1. Field of the Invention
The invention relates to a vehicle power transmission device and, more particularly, to a technology of reducing the shock that occurs when the vehicle power transmission device is switched from a non-driving state to a driving state.
2. Description of the Related Art
There is a known vehicle power transmission device that includes an electrical differential motion portion in which the differential state between the input shaft rotation speed and the output shaft rotation speed is controlled through the control of the operation state of an electric motor linked to a rotating element of a differential motion portion, and a power connection-disconnection device that constitutes a portion of a power transmission path. This vehicle power transmission device is used together with, for example, an engine as a motive power source, to realize a hybrid vehicle.
Through the control of the operation state of the electric motor linked to the rotating element of the electrical differential motion portion, the electrical differential motion portion can be caused to operate in various manners, that is, to output the power supplied to the input shaft from the engine provided as a motive power source directly to the output shaft, or to convert the whole or a portion of the power supplied from the engine into electric energy by using the electric motor as a generator, or to output the power generated by the electric motor alone or together with the power supplied from the engine to the output shaft.
On the other hand, the power connection-disconnection device can be used, for example, in a power transmission device, such as a clutch which is provided, on a power transmission path; a ratio shift portion whose speed change ratio can be changed; a stepped automatic transmission in which a plurality of speed change steps of different speed change ratios can be established by controlling the engagement and release of the engagement elements, such as clutches, brakes, etc.; a continuously variable transmission of a belt type, a toroidal type, etc., which includes a forward-reverse travel switch-over mechanism and whose speed change ratio is continuously changed, etc. Specifically, such a ratio shift portion is normally designed so that the ratio shift portion is switched between a non-driving state in which power transmission is cut off, such as the N (neutral) state, the P (parking) state, etc., and a driving state in which power transmission is performed, such as the D (drive) state, the R (reverse) state, etc. and the switching between the foregoing states is carried out in accordance with a driver's switching operation on a transmission state switch device that includes, for example, a shift lever, a manual shift valve, etc. Then, the power connection-disconnection device connects or disconnects the power transmission, whereby the ratio shift portion is switched between the non-driving state and the driving state, and therefore the vehicle power transmission device as a whole is also switched between the non-driving state and the driving state.
If the state of a motive power source changes when the vehicle power transmission device is switched from the non-driving state to the driving state, there is possibility that the switching to the driving state may not be smoothly carried out or a shift shock may occur. Against this problem, Japanese Patent No. 3346375 discloses a technology of reducing the shift shock by keeping fixed the states of output of the motive power source and the electric motor during the switch of the ratio shift portion from the non-driving state to the driving state.
By the way, according to the hybrid vehicle employing a vehicle power transmission device as described above, the vehicle is run while the engine as a motive power source is kept in an optimum differential state through the control of the operation state of the electric motor linked to the rotating element of the differential motion portion. However, during a low engine efficiency state, for example, when the vehicle is started from a halt state, or the like, the vehicle is run only by the electric motor. Therefore, during the state in which the vehicle is at a stop or the vehicle speed is low, power is not supplied from the engine but is supplied only from the electric motor at the time of a shift operation in which the ratio shift portion is switched from the non-driving state to the driving state, that is, at the time of a garage shift, for example, the N→D shift, the N→R shift, the P→R shift, etc.
However, if the state of charge (SOC) of the electricity storage device provided in the vehicle is low, the engine needs to be driven in order to generate electric power for charging the electricity storage device. Besides, the engine is also driven during the state where warm-up is needed. Such driving of the engine is sometimes performed regardless of the situation of run of the vehicle since the power thereby generated is not for use for running the vehicle. In the case where the foregoing driving of the engine is performed when the ratio shift portion is switched from the non-driving state to the driving state, there is possibility of a change occurring in the input torque to the vehicle driving device and therefore a shift shock being caused.